Merge Cloudef/nix-appimage into this project

[Cloudef]

Allows bundling AppImages with statically linked musl appimage runtime and loader which setups user namespace mount with /nix and also some optional runtime code that can try to construct compatible runtime on non-FHS distros. The loader/runtime part is already here https://github.com/Cloudef/zig2nix/tree/master/src/bundle/loader

Since https://github.com/Cloudef/zig2nix/blob/master/src/bundle/zip.nix already creates the structure of what would be inside the appimage, I think this can be reused.